When a fast moving river meets a slowing moving or still body of water, it deposits its load of sediment in the still or slow moving body of water. It forms a delta. This is how the Mississippi created the Louisiana coastal marshes, this is how the Mississippi created Lake Pepin, which occupies the Upper Mississippi Gorge between Wabasha, Minnesota and Red Wing, Minnesota.
When the Des Moines lobe of the Wisconsin glacier pulled back behind the divide that separates drainage to the Gulf of Mexico from drainage to Hudson Bay, Lake Agassiz pooled between it and the retreating ice sheet. The lake filled to overflowing and spilled over a moraine at Brown’s Valley, Minnesota. River Warren was a torrent that swept the Upper Mississippi Gorge of sediment clear down to bedrock. When River Warren petered out, the Mississippi was reduced to a trickle, an underfit stream, flowing at the bottom of its bedrock canyon.
When its fast-moving tributaries poured into the slow-moving Mississippi, they deposited their loads of sediment in the river and built its floodplain, a maze of islands and sloughs. The Mississippi eroded the islands at their heads and deposited the eroded sediment on the downstream ends, creating a series of C-shaped islands.
The Chippewa River deposited so much sediment in the Mississippi that it dammed the river. Lake Pepin pooled behind the natural dam, and extended clear north to St. Paul, Minnesota.
Then the Minnesota and Mississippi itself began to deposit their sediment and form a delta in the head of Lake Pepin, which it extends south to Red Wing.
The construction of the system of locks and dams, that made the 9-foot navigation channel on the Upper Mississippi, possible stopped the river’s ability to move sediment downstream. A river that moved sediment very slowly, stopped moving it altogether.
But the tributaries did not stop depositing their sediment in the river and what the Minnesota is contributing to Lake Pepin is destroying it. The State of Minnesota has started to determine how much sediment and nutrients the lake can handle. The next step will be to find ways to retain sediment that pours into the Minnesota River and others in the uplands.